Printing plates imageable by ablative discharge and silicone formulations relating thereto

ABSTRACT

Planographic printing plates coated with a two-component silicone formulations that consist of a high-molecular-weight silicone component and a low-molecular-weight silicone polymer. The components are combined in varying proportions with a cross-linking agent to produce compositions of varying viscosities and dispersibilities. The high-molecular-weight component is linear polysiloxane compound with a molecular weight of at least 300,000, and preferably 500,000 or higher. The low-molecular-weight component has a similar structure, with a molecular weight no greater than 70,000. Various substitutions are made to facilitate cross-linking among components. The coatings can contain pigments or other materials to enhance imaging performance.

RELATED APPLICATION

This is a continuation-in-part of Ser. No. 07/616,377, filed on Nov. 21,1990 , now U.S. Pat. No. 5,212,048.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to lithographic printing, and inparticular to plate constructions imageable by ablative discharge aswell as to polymeric silicone coating formulations useful in connectiontherewith.

B. Description of the Related Art

Polyorganosiloxane compounds, or "silicones", can be synthesized in awide variety of forms, and are utilized in numerous commercialapplications. Silicone compounds are based on the repeatingdiorganosiloxane unit (--R₂ SiO--)_(n), where R is an organic radicaland n denotes the number of units in the polymer chain. Each end of thelinear chain is terminated with a functional or non-functional endgroup; the chain may also be "branched" so as to deviate from a strictlylinear structure.

The physical properties of a particular silicone formulation depend onthe length of the polymer chain, the nature of the organic functionalgroups bonded to the silicon atoms, and the terminal groups (moreprecisely, the alpha and omega groups) at each end of the chain. Forexample, the most common silicone compounds are based on thepolydimethylsiloxane unit, --Si(CH₃)₂ O--, which, due to the relativelysmall organic content of the chains, have a limited range ofcompatibility with organic compounds. By contrast, silicones containingaryl functional groups tend to exhibit properties more commonlyassociated with organic materials, and such silicones are generallymiscible with a broader range of such materials.

Many polymeric silicone compounds exhibit rubber-like characteristics.These compounds are employed in numerous industrial and commercialenvironments as a direct substitute for natural rubber. "Siliconerubber" is typically prepared from "silicone gum", which connotes aviscous, high-molecular-weight polydiorganosiloxane compound, bycross-linking (or "curing") the polymer chains. It is often convenientto form silicone rubber in situ on a surface by applying components thatwill form the rubber and subsequently curing these. Curing (orcross-linking) permanently immobilizes the polymeric components of therubber by establishing chemical linkages between them.

Curing can be accomplished in a number of ways, but generally depends onthe presence of reactive functional groups on the polymer chains thatinteract and bond with one another. "Condensation cure" reactions referto those in which a small molecule is eliminated when the two functionalgroups are joined. Typical condensation-cure reactions in siliconechemistry involve combination of silanol functional groups with othersuch groups to produce an oxygen linkage with the elimination of water."Addition cure" reactions involve no loss of species, and can involve,for example, hydrosilylation reactions between olefinic functionalgroups (such as vinyl) and hydrosiloxane groups.

Variations on the traditional condensation-cure reaction include the"moisture-cure" approach, in which a precursor functional group is firsthydrolyzed to form a reactive hydroxyl-bearing group, which thencombines with a silanol group as discussed above. Suitable precursorcompounds include acetoxy, alkoxy and ketoxime functional silanes, whichform acid, alcohol and ketoxime byproducts, respectively, uponhydrolysis.

Silanol-functional silicones and mixtures of silanol-functionalsilicones with silicones containing acetoxy, alkoxy or ketoxime groupsare relatively stable so long as moisture is excluded; this isparticularly true for silicone polymers having appreciable molecularweights. Obtaining useful reaction rates generally requires a catalystsuch as a metal carboxylate compound.

Silanol groups also react with hydrosiloxane species to liberatehydrogen and produce the silicon-oxygen-silicon linkage characteristicof the condensation cures. Use of a metal salt catalyst (such asdibutyltindiacetate) is generally necessary to achieve useful reactionrates. Because it proceeds rapidly when catalyzed, this reaction iswidely used for silicone coating formulations applied on a coating lineto a web substrate.

In addition to these mechanisms, silicone polymers are sometimescross-linked using multifunctional acrylate or methacrylate monomers.The polymers are exposed to an electron beam or combined with aphotoinitiator species and then exposed to actinic radiation in order toproduce free-radical derivatives that combine with one another. Otherapproaches to cross-linking are described in U.S. Pat. No. 4,179,295,the entire disclosure of which is hereby incorporated by reference.

The reactive, cross-linking functional groups can be incorporated at thetermini of a polymer chain, or at a desired frequency within a copolymerchain. In order to achieve the elastomeric properties associated withsilicone rubber, large polymeric units ("base polymers") arecross-linked by smaller oligomers or multifunctional monomers.Frequently, this is accomplished by providing the base polymers with onetype of functional group, and incorporating the complementary functionalgroup on the cross-linking molecules. For example, the addition-curereaction described above can be utilized to produce elastomericcompositions by combining base polymer or copolymer chains that containolefinic functional groups with small cross-linking molecules that havehydrosiloxane-functional termini.

Silicone rubber coatings have been adopted by some manufacturers ofplanographic printing plates. Planographic printing, as contrasted withletter-press and gravure printing, relies on plate constructions inwhich image and non-image areas lie substantially in the same plane. Theplate is prepared by altering the affinities of different areas of theplate for ink. Depending on the type of plate imaging system employed,non-image plate areas become (or remain) oleophobic, or ink-repellent,while image areas remain (or become) oleophilic, or ink-accepting. Inkapplied to the plate surface, e.g., by a roller, will adhere to theoleophilic image areas but not the oleophobic non-image areas. The inkedplate is then applied to the recording medium (in direct printing) or toan intermediate "blanket" cylinder which then transfers the image to therecording medium (in offset printing).

Manufacturers of planographic printing plates often employ siliconerubber compositions as plate coatings due to their oleophobic character,which provides compatibility with conventional planographic printingtechniques. Silicone coatings are commonly used in conjunction withso-called "dry" plates. In contrast to the traditional "wet" plate,which requires application of a fountain or dampening solution to theplate prior to inking in order to prevent ink from adhering to andtransferring from non-image areas, the non-image material of dry platesis itself sufficiently ink-repellent that no fountain solution isnecessary.

One hypothesis explains this effect as arising from interaction betweenthe non-image component of a dry plate and the (usually aliphatic)solvent or solvents employed in printing inks, resulting in theformation of a thin layer of solvent on the surface of the non-imagecomponent. Like the fountain solution of a wet plate, this surface layeracts as a boundary and prevents the ink from adhering to the plate.

Blank dry plates are subjected to an imaging process that removes thesilicone coating from image areas to reveal an oleophilic surface.Imaging can be accomplished in a number of ways. Photosensitizationmethods rely on incorporation of a photoresist material in the platestructure which, upon exposure to radiation (e.g., visible light),alters the solubility or anchorage properties of the silicone. Intypical commercial plates, exposure to light results either in firmanchorage of the silicone coating to the plate (in positive-workingplates) or in destruction of the existing anchorage (in negative-workingplates). Depending on the process chosen, the plate is first exposed toactinic radiation passing through a positive or negative rendition ofthe desired image that selectively blocks transmission of the radiationto the plate. After this exposure step, the plate is developed inchemical solvents that either anchor the exposed silicone or remove itto produce the final, imaged plate.

For example, a number of photosensitive dry-plate constructions arecurrently known and used in the art. In one approach, the photosensitivematerial is combined with the silicone coating prior to its applicationonto a substrate. Another construction relies on incorporation of thephotosensitive compound within an underlying layer, exposure eitherweakening or strengthening the bond between layers. See, e.g., U.S. Pat.Nos. 3,511,178 and 4,259,905. In a third alternative, ink-acceptingtoner particles are fixed to the silicone surface according to thedesired image pattern.

Plates can also be imaged by means other than photoexposure, e.g., byablation using spark-discharge apparatus (such as that described in U.S.Pat. No. 4,911,075, the entire disclosure of which is herebyincorporated by reference) or laser-discharge apparatus (such as thatdescribed in Ser. No. 07/917,481, the entire disclosure of which ishereby incorporated by reference) that utilize electronic signals tolocate and produce a discharge at the precise positions on the platewhere the silicone coating is to be removed to reveal an underlyingoleophilic surface. The spark-discharge apparatus can make contact withthe plate or be held at a relatively fixed distance above the plateduring the imaging process.

Silicone compositions used as coatings for planographic printing platestypically include two basic constituents, namely, a primarypolyorganosiloxane base-polymer component and a smaller cross-linkingcomponent. The base component is usually a linear, predominantlypolydimethylsiloxane copolymer or terpolymer containing unsaturatedgroups (e.g., vinyl) or silanol groups as reactive centers for bondingwith the cross-linking molecules. These groups are commonly situated atthe chain termini; alternatively, it is possible to utilize a copolymerincorporating the reactive groups within the chain, or branchedstructures terminating with the reactive groups. It is also possible tocombine linear difunctional polymers with copolymers and/or branchpolymers. See, e.g., published Japanese Patent Applications 1-118843 and1-179047.

The cross-linking component is generally a multifunctional, monomeric oroligomeric compound of low molecular weight, which is reacted with thefirst component to create connections among the chains thereof. Thecuring reaction generally requires some type of catalyst, eitherchemical or physical, to produce favorable kinetics. Platinum metalcomplexes (such as chloroplatinic acid) are often employed to facilitateaddition cures, while metal salt catalysts (such as adialkyltindicarboxylate) are frequently used in conjunction withcondensation cures.

If the functional groups of the cross-linking component are situated atthe chain termini, cross-linking molecules will form bridges among thebase-polymer molecules (particularly if the latter have functionalgroups distributed along the chains). On the other hand, if thecross-linking component contains functional groups distributed along itslength, each molecule will form numerous points of attachment with thebase-polymer molecules. Typically, this type of cross-linking moleculeis combined with base polymers having chain-terminal functional groupsin order to maximize the number of different base-polymer moleculesattached to each cross-linking chain.

Modifiers can be added to alter physical properties, such as adhesion orrheology, of the finished coating. One can also add colorants, in theform of dyes or pigments, to the silicone formulation to facilitatequality-assurance evaluation or monitoring of the photoexposure process.Pigments and/or dyes can also be used to enhance imaging performance inplates that will be imaged by ablation using discharge apparatus, asfurther described herein.

Current silicone coating formulations suffer from a number ofdisadvantages, some stemming from physical characteristics of thepolymer system itself, and others arising from the requirements ofavailable coating apparatus. Silicone coatings are generally prepared bycombining a silicone polymer with a solvent (usually aliphatic) and,possibly, other volatile components to control viscosity and assist indeposition. Because the solvent evaporates after the coating is applied,the amount of silicone actually deposited per unit surface area dependson the relative proportions of silicone and volatile components; thisproportion is referred to as the "solids content" of the composition,and is typically expressed as a percentage. Too little solvent canproduce a coating that is difficult to apply, while excessiveproportions of solvent can result in deposition of too little of theactual silicone material during coating, thereby reducing platedurability. The relatively low-weight polymers currently employed forproducing printing plates tend to exhibit low solution viscosities;because of the necessity of preserving a minimum viscosity level forcoating purposes, this characteristic limits the extent to whichlow-weight silicones can be diluted with solvent to control thedeposition rate. Furthermore, these coatings also tend to require anarrow range of solids content for uniform application, a constraintthat results in further limitation of the ability to vary dilution.

Finally, low-weight silicone compositions form poor dispersions withsolid particles. For a growing number of platemaking applications,introduction of pigments or other particles is necessary for optimumplate performance. Not only is it difficult initially to disperseparticles in low-weight silicones, but over time the particles thathave, in fact, been dispersed tend to reagglomerate. Moreover, theabove-noted problems involving low solution viscosities and narrowsolids-content requirements become accentuated when particles areintroduced into low-weight compositions.

A silicone coating composition is applied to a plate substrate using anyof a variety of well-known coating techniques. The choice of techniqueis critical not only to the ultimate performance of the plate, but alsoto the efficiency and reliability of the overall platemaking process.Typical coating techniques include roll coating, reverse-roll coating,gravure coating, offset-gravure coating, and wire-wound rod coating. Thecoating procedure must be rapid enough to achieve a satisfactoryproduction rate, yet produce a highly uniform, smooth, level coating onthe plate. Even small deviations in coating uniformity can adverselyaffect plate performance, since the planographic printing processdepends strongly on coplanarity of image and non-image areas; in otherwords, the printing pattern reproduced by the plate must reflect theconfiguration of oleophilic and oleophobic areas impressed thereon, andremain uninfluenced by topological characteristics of the plate surface.

Because the physical properties of a given silicone formulation can bevaried only to a limited extent by the use of solvents and modifiers(especially in the case of low-weight silicones), particular coatingformulations tend to favor use in conjunction with a particular type ofcoating line. For example, addition-cure coatings having 100% solidscontent are most advantageously applied using offset gravure-typecoating equipment. For formulations having low viscosities (whichtypically imply low solids contents), roll-coating and rod-coatingapplications are preferred.

However, as a practical matter, the number of coating lines available toa particular manufacturer is likely to be limited. It may thereforeprove impossible to utilize a particular silicone formulation withreadily available coating technology, forcing plate manufacturers tochoose formulations based on compatibility with their coating linesrather than optimal performance for a given application.

This limitation can prove appreciable, since coating properties requiredfor a particular application may narrow the range of acceptableformulations. Some coating properties, such as durability, can dependnot only on the silicone formulation or solids content, but also on thesurface to which the coating is applied and the environment in which itis used. Other characteristics, such as the ability of the siliconematrix to accept and retain dispersions of large amounts of particulatematerial (a particularly important feature for spark-dischargeplanographic applications), can rule out entire classes of formulationsand/or severely limit the number of coating techniques that may beemployed.

DESCRIPTION OF THE INVENTION A. Objects of the Invention

It is, therefore, an object of the present invention to provide novelsilicone formulations containing two base polymers, the proportions ofwhich can be varied to control the viscosity of the coating compositionand the physical properties of the cured coating.

It is another object of the present invention to provide siliconeformulations into which relatively large amounts of particulate materialcan be readily dispersed and maintained as a stable dispersion.

It is a further object of the invention to provide silicone formulationsthat may be uniformly applied to a planographic printing plate with goodcontrol of the amount of coating actually deposited on a surface.

It is still another object of the invention to provide

planographic printing plate constructions having silicone coatingscontaining two base polymers, the proportions of which can be varied tocontrol the viscosity of the coating composition and the physicalproperties of the cured coating on the plate.

Still a further object of the invention is to provide planographicprinting plate constructions having silicone coatings that containrelatively large amounts of particulate material dispersed therein.

Other objects will, in part, be obvious and will, in part, appearhereinafter. The invention accordingly comprises the compositions,features of construction, combination of elements, arrangement of parts,and relations of process steps that will be exemplified in the followingdescription, and the scope of the invention will be indicated in theclaims.

B. Summary of the Invention

The compositions of the present invention comprise silicone systemshaving two primary components, a high-molecular-weight silicone gum anda distinctly lower-molecular-weight silicone polymer. These componentsare combined in varying proportions with a suitable cross-linking agentto produce compositions of varying viscosities, and gooddispersibilities and dispersion stability.

As used herein, the term "base polymer component" refers to one of theprimary polymers. The fact that we describe compositions having morethan one base polymer component should not lead to confusion withdescriptions of traditional silicone coating formulations, in which a"two-component" system denotes a composition having a single basepolymer and a cross-linking agent. Furthermore, traditional compositionsalso contain modifiers that promote adhesion or otherwise alter physicalproperties, and these additives also should not be confused with asecond silicone component.

The advantages offered by the present invention stem, in part, from theuse of a high-molecular-weight gum stock as a primary system component.Silicone gums having high dimethylsiloxane content can be used to createcoatings that resist adhesion (that is, exhibit the property sometimesreferred to as "abhesion") to compounds such as ink. We have also foundthat silicone-gum based polymeric systems are amenable to stabledispersion with solid particulates and can be applied with greateruniformity using a variety of coating lines as compared withlower-weight compositions. The former feature stands in marked contrastto polymers prepared from lower-weight silicones; as discussed above,such compositions are difficult to use the basis for dispersions.

On the other hand, silicone compositions prepared solely from gum stocktend to lack durability, particularly in the demanding environment ofplanographic printing. Our experience indicates that coatings preparedsolely from the lower-weight polymers perform more durably, althoughthese compositions tend to suffer from uniformity and dispersibilitylimitations. Furthermore, the viscosities of these low-molecular-weightpolymers can be varied only within a limited range.

By combining these two components, we are able to achieve, in a singleformulation, the advantages of compositions prepared from silicone gumsand those prepared from lower-weight polymers, while minimizing thedisadvantages associated with either type of composition individually.Specifically, we are able to disperse (and retain as dispersions)relatively large amounts of solids into our compositions, and controlboth system viscosity and durability by varying the proportions of thehigh-molecular-weight gum component (which exhibits high solutionviscosity) and the low-molecular-weight component (which exhibits lowsolution viscosity and better durability characteristics).

In one embodiment of the invention, we vary the positions of eachcomponent's cross-linking constituents. For example, readily availablegum compositions have functional (cross-linking) groups distributedalong the chain, while it is possible to obtain lower-weight polymerswith functional groups limited to the chain termini. Placement ofidentical functional groups at different positions within the molecularstructures of the two components allows us to produce polymer systemsconsisting of two integrated networks that each combine differently withthe cross-linking molecules, thereby contributing to the properties ofthe bulk material in different ways.

In another embodiment, we provide each component with a differentfunctional group or groups, thereby retaining independent control overeach component's interaction with cross-linking molecules. Thecross-linking component can consist of a single molecular species thatcontains two different functional groups, each complementary to thefunctional group associated with one of the base polymers. The positionsof the functional group within each base polymer and within thecross-linking molecules can also be varied to achieve different physicalproperties.

Utilizing different functional groups for each component allows us tocontrol the extent, rate and timing of cure for each componentindependently of the other, since each component cross-links by adifferent chemical reaction. This "dual cure" capability allows us tocombine, in one formulation, physical properties normally associatedwith a single type of cross-linking mechanism; it also affords greaterflexibility in the choice of a coating deposition technique.

C. Detailed Description of the Preferred Embodiments 1. Constituents

The first base-polymer component of the compositions of the presentinvention is a high-molecular-weight polysiloxane compound with amolecular weight of at least 300,000, and preferably 500,000 or higher.The structure of this compound is generally linear. While it is possibleto introduce a minor amount of branching, significant deviation from alinear structure decreases desirable properties such as good flow andelastomeric characteristics.

This first component preferably comprises a chain of dimethylsiloxanegroups, [--(CH₃)₂ SiO--]_(n). However, we have also obtainedadvantageous results with copolymers of dimethylsiloxane and highermethylalkysiloxanes having up to 18 carbons in the alkyl group (largeralkyl groups tend to adversely affect ink-release and durabilityproperties of the finished coating); phenylmethylsiloxanes;diphenylsiloxanes; and 3,3,3-trifluoropropylmethylsiloxane. Phenylmethyland diphenylsiloxanes tend to confer heat tolerance, while the trifluorocompounds offer greatest resistance to most of the aliphatic andaromatic solvents that are commonly found in printing inks. However, topreserve favorable ink-release properties, the proportion ofdimethylsiloxane to other functional groups is preferably at least 70%.In the case of trifluoropropylmethylsiloxane, the proportion thereof todimethylsiloxane can range up to 80%, while for phenylmethylsiloxane ordiphenylmethylsiloxane groups the proportion to dimethylsiloxanepreferably does not exceed 20%.

The first base-polymer component preferably contains functional groupsrandomly distributed along the polymer chain (and, if desired, aschain-terminating groups as well). Although in some cases it may bepossible to utilize functional groups located solely at the chaintermini of this component, we would expect to obtain less advantageouscross-linked structures using such compounds.

The functional group is introduced by incorporating substituted siloxanemonomers into the polymerization reaction mixture, according to methodswell-known in the art. The frequency of cross-linking (and, hence, theextent of curing) in the coating into which this component is introducedgenerally depends on the ratio of siloxane units containing functionalgroups to those which do not.

It is also possible to alter the physical properties of the cross-linkedpolymer while maintaining a constant mole percentage of functionalgroups by utilizing disubstituted monomer units, thereby introducing twofunctional groups at a given chain position. Assuming the substitutedmonomers are randomly distributed along the polymer chain, this strategyresults in greater spacing between the cross-linking sites but alsocreates locally higher cross-link density (because two functional groupsare free to react) at each cross-linking position. Polymers producedusing this disubstituted approach yield coatings that exhibitelasticities similar to those based on monosubstituted polymers having asimilar weight percentage of functional groups (the greater spacingbetween reactive monomer units and locally higher cross-link densitiestending to balance one another), and therefore retain good durabilitycharacteristics. Such compositions are particularly useful in thepreparation of printing plates that employ flexible-film substrates,where elasticity of the coating is critical.

The second base-polymer component of our formulations is a relativelylight polysiloxane compound with a molecular weight up to 70,000 but inexcess of 5,000, with molecular weights in the range 40,000-50,000 beingpreferred. The structure of this compound is preferably substantiallylinear, although once again it is possible to introduce a minor amountof branching. The functional-group characteristics described above withregard to the first component apply as well to this second component.However, it is preferable to have the functional groups in this secondcomponent situated at the ends of the polymer chains. This positioninghas been found to promote durability, while random distribution alongthe chain produces distinctly less elastomeric compositions incombination with the first base-polymer discussed above.

The relative proportions of the primary and secondary components can bevaried over a wide range, allowing significant variation in theviscosities, dispersibilities, dispersion stabilities and solidscontents of the final coatings obtainable using the present invention.Our preferred working range allows variation in the relativeproportions, in weight percent, of each component from 10% to 90% (i.e.10% first component and 90% second component to 90% first component and10% second component).

The cross-linking species of the present invention is preferably amonomeric or oligomeric polysiloxane that contains functional groupscomplementary to those of the base-polymer components. Preferably, themolecular weight of the cross-linking agent is between 2,000 and 5,000,although cross-linkers with molecular weights of 500 or even less may beusable under some circumstances. However, for thermal curing in an openenvironment, if the molecular weight of the cross-linking species is toolow excessive amounts will evaporate from the reaction mixture.

For thermal curing, the fastest (and therefore most practical) curetimes are obtained with relatively large numbers of functional groupsdistributed along the chains of the cross-linking component. However,while cross-linking molecules containing only terminal functional groupsrequire significant reaction times, the compositions produced therewithtend to exhibit excellent elastomeric properties. Accordingly, ourpreferred route use a limited number of functional groups spaced aswidely along the cross-linking chains as possible, the precise numbersand configuration being dictated by the user's reaction-timeconstraints.

2. Reactive Functional Groups

A wide range of complementary functional groups, and hence curingmechanisms, can be used with the present invention. For addition-curecoatings, our preferred functional group is vinyl; however, otherα-olefinic groups such as allyl (1-propenyl) and 1-butenyl groups aralso readily employed in the two major components. Alternatively, it ispossible to use higher alpha olefins up to 18 carbons as functionalgroups for addition-cure cross-linking. Such groups are advantageouslyemployed in a hydrosilylation reaction in which asilicon-hydride-functional cross-linking species, such as apolyhydrosiloxane (e.g., a polymethylhydrosiloxane polymer, copolymer orterpolymer), or a polyfunctional polysilane, is employed. Hereafter, theterm "silicon hydride" will be used generally to refer to species of theform R₃ SiH, where each R independently represents an organic orinorganic atom or moiety covalenty bound to the silicon atom.

We also employ functional groups that are cross-linked bycondensation-cure mechanisms (or the moisture-cure variation). Forcondensation-cure systems, the two major components contain silanolgroups (i.e. hydroxyl functionalities), and the cross-linking componentis silicon-hydride functional. Once again, suitable cross-linkingspecies include polymethylhydrosiloxane polymers, copolymers andterpolymers, and polyfunctional polysilanes. It is also possible toutilize functional groups curable by ultraviolet or electron-beamradiation (either directly or via activated photoinitator species),e.g., acrylate, methacrylate or cycloaliphatic-epoxy species.

Preferably, the functional groups of the two major components arepresent on the polymer chains in amounts not exceeding 1.0 mole percent,with the range 0.1 to 0.5 mole percent being especially preferred; inthe case of vinyl functionalities, the especially preferred range is 0.1to 0.3 mole percent.

It is not necessary for all functional groups to be identical within aparticular polymeric composition, or even within a single component. Bycombining functional groups, we can exert control over the polymernetworks that result from curing and thereby improve selected propertiesof a formulation (e.g., expansion of the range of substrates to whichthe formulation can be applied). For example, both base-polymercomponents of a single formulation may include 75% vinyl and 25% alkoxygroups. After combining the base polymers with a silicon-hydridefunctional cross-linker, one can first expose the polymer toaddition-cure reaction conditions and then allow post-cross-linkingexposure of the coating to ambient air to trigger subsequent moisturecure (i.e. reaction of the alkoxy groups to produce additionalcross-linking and bonding to the substrate).

Alternatively, one can confine each of the vinyl and alkoxy functionalgroups to a single base-polymer component. Cross-linking thevinyl-functional base-polymer component results in an apparently solidcoating matrix; while entrained therein, the second component remainschemically independent, and exposing the system to moisture results information of a second, interpenetrating polymer network. (It should beunderstood that the segregation or combining of functional groups amongbase-polymer components represents a consideration separate from thechoice of where to position such functional groups along the chains.)

Use of a dual-cure approach can facilitate simultaneous exploitation ofbeneficial properties associated with two types of cross-linkingmolecules. For example, vinyl or other lowolefin groups tend to promotegood abhesion properties, while acrylate and methacrylate cross-linkagestend to limit the degradative effects of solvent attack from printinginks. By combining these two groups within the first base-polymercomponent, we are able to produce dry-plate coatings that repel inkwhile resisting attack from the very solvents that are probablyresponsible for the ink-repellent behavior. Furthermore, because thefree-radical linkage mechanism of the acrylate and methacrylate groupsdo not interfere with or participate in the hydrosilylation reactionthat cross-links the vinyl and silicon hydride groups, these reactionsare compatible with one another. It is also possible to utilizeunmodified silicon hydride cross-linking molecules with this dual-curereaction, since the acrylate and methacrylate groups react with oneanother directly (in the presence of suitable initiators).

Dual-cure coatings find particularly advantageous application tomulti-layer printing-plate constructions such as those described in the'481 application. In these constructions, a high degree of intercoatadhesion between the near-IR absorbing layer and the overlying siliconelayer is desirable. This can be accomplished with a dual-cure approachwherein one of the two main silicone components is acrylate- ormethacrylate-functional (an acrylate-functional second component beingparticularly preferred) and further wherein the underlying layer alsocontains an acrylate- or methacrylate-functional species. Typically, theunderlying layer is first applied and dried, taking care to avoidconditions that would prematurely react the acrylate- ormethacrylate-functional components. The dual-cure silicone coating isthen applied and the first curing reaction initiated; in a preferredembodiment, this first cure is a hydrosilylation betweenvinyl-functional groups of one of the two base-polymer components and apolymethylhydrosiloxane cross-linking component. The second curingreaction involving the acrylate- or methacrylate-functional groups isthen initiated (ordinarily by exposure to an ultraviolet orelectron-beam source and, in the case of ultraviolet exposure, in thepresence of a suitable photoinitiator). The acrylate- ormethacrylate-functional species in the underlying layer will alsoundergo reaction at this time, resulting in the formation of chemicalbonds between, as well as within, the two layers.

Alternatively, the underlying layer can contain free vinyl or otherα-olefinic species for reaction silicon hydride groups of thecross-linker, which also reacts with similar groups on at least one ofthe base-polymer components.

Preferred functional groups for applications in which the base-polymercomponents share at least one common functional group are those in whichthe common functional group is unreactive with itself but reactive withthe cross-linker, or reactive with itself but unreactive with thecross-linker. In the former case, for example, the common group can bevinyl or other α-olefinic unsaturated groups with a silicon hydridecross-linker, or silicon hydride functional groups with a cross-linkerbearing α-olefinic unsaturated groups. An example of the latter case isuse of acrylate- or methacrylate-functional groups as the commonspecies, with a cross-linker that combines with other base-polymerfunctional groups via addition cure or condensation cure.

When each base-polymer component bears a different functional group orgroups, these should also be chosen so as to be unreactive with oneanother; however, at least one of the functional groups must react withthe cross-linker. Preferred formulations include an addition-cure firstcomponent and cross-linker (e.g., where the first component containsvinyl groups and the cross-linker is a polyhydrosiloxane, or vice versa)and an acrylate- or methacrylate-functional second component; or acondensation-cure (or moisture-cure) first component and cross-linker(e.g., where the first component and cross-linker both contain silanolgroups, or where one of the first component and cross-linker containssilanol groups and the other contains polyhydrosiloxane groups).

3. Loading with Pigments

If pigments or other solids are to be introduced into the finalformulation, these can be dispersed within one of the components(preferably the high-molecular-weight first component) prior tocombination of the two components, or within the final formulation aftercombination.

Although the formulations of the present invention can accommodate alarge variety of pigments, preferred material are those that can enhancethe response of a printing plate to ablative discharge. Particularlypreferred pigments are described in U.S. Pat. Nos. 5,109,771 and5,165,345, and U.S. application Ser. No. 07/894,027, the entiredisclosures of which are hereby incorporated by reference.

4. Enhancing Absorption Characteristics

For a number of printing applications, it is useful to enhance theabsorbence characteristics of a silicone coating with respect to certainkinds of radiation. This capability becomes important, for example, inconnection with laser-based imaging systems. We have developed a numberof approaches to enhance absorbence in one important spectral region,namely, the near-infrared ("near-IR") wavelength spectrum.

The simplest technique for enhancing absorbence of a coating is todisperse, within the polymer, compounds having appropriate responsecharacteristics. The pigments discussed in Examples 5-11 below allperform well as near-IR absorbers.

Alternatively, chemical groups that exhibit near-IR absorbence can beincorporated within the backbone of the first and/or second base-polymercomponents. The absorptive strength of the chemical group and the numberof such groups introduced into the backbone determine the overallabsorbence of a silicone component. Siloxy compounds containingphthalocyanines or naphthalocyanines, for example, can be used asmonomers in the synthesis of either or both base-polymer components.Naphthalocyanines suitable for incorporation in this manner aredescribed in U.S. Pat. Nos. 4,977,068, 5,047,312, 5,039,798 and5,023,168; analogous compounds that make use of phthalocyanine moietiescan also be utilized advantageously. Thus, as an illustration, thenaphthalocyanine species discussed in the '312 patent noted abovecomprise alkoxy-substituted silicon atoms bound to thenaphthalocyanine-bearing silicon atom; such molecules may becopolymerized, in desired proportions, typically with other silanes orother functional silicone oligomers via the hydrolytic process,resulting in polymers that may be used as a component of the presentinvention.

It is also possible to use any of these compounds as additives to one orboth base-polymer component through dispersion (rather than chemicalincorporation) for simplicity.

5. Planographic Printing-Plate Constructions

The compounds of the present invention can be used as coatings in a widevariety of printing-plate constructions. Particular advantage is gainedin connection with plates imaged by ablative discharge. Examples of suchplates are given in the '027 and '481 applications, in allowedapplication Ser. No. 07/741,099 (the entire disclosure of which ishereby incorporated by reference), and in the '771 and '345 patents.

Such plates typically consist of a durable, oleophilic base material, anablative layer (e.g., metal in the case of spark-discharge plates or aradiation-absorbing layer in the case of laser-imaged plates), and anoleophobic coating. Herein the term "substrate" is used in a genericsense to connote the plate layer or layers to which the oleophobicsilicone coatings of the present invention are applied.

6. Examples

We will now describe preparation of several representative formulations.

EXAMPLES 1-8

In each of these eight examples, a pigment was initially dispersed intothe high-molecular-weight gum component, which was then combined withthe second component. For the gum component, we utilized a linear,dimethylvinyl-terminated polydimethylsiloxane supplied by Huls America,Bristol, Pa. under the designation PS-255. For each formulation, the gumcomponent was combined with one of the following pigments:

    ______________________________________                                        Pigment  Trade Name   Supplier                                                ______________________________________                                        ZnO      Kadox 911    Zinc Corp. of America                                                         Monaca, PA                                              Fe.sub.3 O.sub.4                                                                       BK-5000      Pfizer Pigments, Inc.                                                         New York, NY                                            SnO.sub.2 -based                                                                       CPM 375      Magnesium Elektron, Inc.                                                      Flemington, NJ                                          SnO.sub.2 -coated                                                                      ECP-S        E.I. duPont de Nemours                                  hollow   Micronized   Wilmington, DE                                          sphere                                                                        TiC      Cerex        Baikowski International                                                       Corp., Charlotte, NC                                    Heliogen L 8730       BASF Corp.                                              Green                 Holland, MI                                             WO.sub.2.9                                                                             Tungsten     Cerac Inc.                                                       Oxide Powder Milwaukee, WI                                           MnO.sub.2                                                                              Manganese    Kerr-McGee Chemical Corp.                                        Dioxide Powder                                                                             Oklahoma City, OK                                       ______________________________________                                    

The first four pigments have been found to be well-suited to non-contactspark-discharge imaging applications, while the next four provide goodnear-IR absorbance.

Each pigment was used to prepare a different formulation. First,pigment/gum dispersions were prepared by combining 50% by weight of eachpigment and 50% by weight of the gum in a standard sigma arm mixer.

Next, the second component was prepared by combining 67.2% by weight ofthe mostly aliphatic (10% aromatic content) solvent marketed by ExxonCompany, USA, Houston, Tex. under the trade name VM&P Naphtha with 16.9%of the vinyl-dimethyl-terminated polydimethylsiloxane compound marketedby Huls America under the designation PS-445, which contains 0.1-0.3%methylvinylsiloxane comonomer. The mixture was heated to 50-60 degreesCentigrade with mild agitation to dissolve the PS-445.

In separate procedures, 15.9% by weight of each pigment/gum dispersionwas slowly added to the dissolved second component over a period of 20minutes with agitation. Agitation was then continued for four additionalhours to complete dissolution of the pigment/gum dispersions in thesolvent.

After this agitation period, 0.1% by weight of methyl pentynol was addedto each blend and mixed for 10 minutes, after which 0.1% by weight ofPC-072 (a platinum-divinyltetramethyldisiloxane catalyst marketed byHuls) was added and the blends mixed for an additional 10 minutes. Themethyl pentynol acts as a volatile inhibitor for the catalyst. At thispoint, the blends were filtered and labelled as stock coatings ready forcross-linking and dilution.

To prepare batches suitable for wire-wound-rod or reverse-roll coatingapplications, the stock coatings prepared above were each combined withVM&P Naphtha in proportions of 100 parts stock coating to 150 parts VM&PNaphtha; during this step, the solvent was added slowly with goodagitation to minimize the possibility of the solvent shocking (andthereby disrupting) the dispersion. To this mixture was added 0.7 partsPS-120 (a polymethylhydrosiloxane cross-linking agent marketed by Huls)under agitation, which was continued for 10 * minutes after addition toassure a uniform blend. The finished coatings were found to have a potlife of at least 24 hours, and were subsequently cured at 300 degreesFahrenheit for one minute.

EXAMPLES 9-11

In each of these next examples, commercially prepared pigment/gumdispersions were utilized in conjunction with a second,lower-molecular-weight second component. The pigment/gum mixtures, allbased on carbon-black pigment, were obtained from Wacker SiliconesCorp., Adrian, Mich. In separate procedures, we prepared coatings usingPS-445 and dispersions marketed under the designations C-968, C-1022 andC-1190 following the procedures outlined above (but omitting thedispersing step). The following formulations were utilized to preparestock coatings:

    ______________________________________                                        Order of Addition                                                                         Component       Weight Percent                                    ______________________________________                                        1           VM&P Naphtha    74.8                                              2           PS-445          18.0                                              3           Pigment/Gum Disperson                                                                         7.0                                               4           Methyl Pentynol 0.1                                               5           PC-072          0.1                                               ______________________________________                                    

Coating batches were then prepared as described above using thefollowing proportions:

    ______________________________________                                        Component       Parts                                                         ______________________________________                                        Stock Coating   100                                                           VM&P Naphtha    100                                                           PS-120 (Part B) 0.6                                                           ______________________________________                                    

The three coatings thus prepared were found to be similar in cureresponse and stability to Examples 1-8.

EXAMPLE 12

The formulations described above undergo cross-linkage by addition-curehydrosilylation reactions. It is possible to replace theolefin-functional groups with silanol groups to facilitate acondensation-cure mechanism, even utilizing the same PS-120cross-linking agent. One approach to condensation cure is use of alkoxyfunctional groups to facilitate cure by exposure to moisture.

Dispersion of a pigment into a silanol-functional high-molecular-weightpolymer is accomplished in the same manner as described in Examples 1-8.If PS-445 is retained as the low-molecular weight component, dual-cureis possible; replacing the olefin-functional PS-445 with asilanol-functional (or alkoxy-functional) silicone results in asingle-cure reaction analogous to those described above.

Cross-linking to the silanol-functional groups is accelerated by acatalyst such as a dialkyltindicarboxylate (e.g., dibutyltindiacetate),a metal carboxylate (e.g., zinc dioctoate), a titanate, or a thermallyactivated latent catalyst (e.g., as described in published EuropeanPatent Application 338,947). None of these catalysts would be expectedto interfere with the addition-cure components described above, therebyfacilitating a dual-cure system. With this approach, theolefin-functional secondary component is subjected to the cross-linkingconditions described in Examples 1-8 before silanol cross-linking.

EXAMPLE 13

We compared the solids content of an unpigmented coating prepared usingour two-component approach to an unpigmented coating having similarviscosity but based entirely on the low-molecular-weight PS-445. Bothcoatings were prepared using the techniques outlined in Examples 1-8,but omitting the pigment-dispersion step. The proportions of reactantsutilized in each of the coatings were as follows:

    ______________________________________                                                       Weight Percent                                                 Component        Coating 1                                                                              Coating 2                                           ______________________________________                                        VM&P Naphtha     90.63    83.9                                                PS-445           4.5      15.5                                                PS-255           4.5      --                                                  Methyl Pentynol  0.05     0.05                                                PC-072           0.05     0.05                                                PS-120           0.27     0.5                                                 ______________________________________                                    

Coatings 1 and Coating 2 exhibit viscosities and can be applied usingthe same coating technique to yield similarly uniform coatings. However,the solids content of Coating 2--which represents a typicalsingle-component approach--is approximately twice that of Coating 1.Furthermore, because both the viscosity and solids content of Coating 2depend entirely on the proportion of a single silicone comopenent,coating viscosity requjirments impose a significant lijmtaion on thedegree to which solids content can be altered.

By contrast, the formuation of Coating 1 can be altered by changing therelative proportions of PS-445 and PS-255, retaining a consistentviscosity while varying solids content. This is shown in the followingtwo variations of Coating 1, one (Coating LS) formulated for low-solidscontent, and the other (Coating HS) formulated for high-solids content.

    ______________________________________                                                      Weight Percent                                                  Component       Coating LS                                                                              Coating HS                                          ______________________________________                                        VM&P Naphtha    92.7      84.45                                               PS-445          1.0       13.0                                                PS-255          6.0       2.0                                                 Methyl Pentynol 0.05      0.05                                                PC-072          0.05      0.05                                                PS-120          0.2       0.45                                                ______________________________________                                    

After solvent evaporation, Coating LS will deposit less than half theweight of silicone that would be deposited by Coating HS.

EXAMPLE 14

We modified Coating 1 of the previous example to obtain adiphenylsiloxane composition by replacing the PS-445 second componentwith an equivalent weight percentage of PS-767.5, also marketed by Huls.PS-767.5 is a vinyl-terminated 4-6%-diphenylsiloxane, 94-96%dimethylsiloxane copolymer.

The physical characteristics of this coating were similar to those ofExamples 1-8, but would be expected to exhibit greater heat tolerance.

EXAMPLE 15

We modified Coating 1 of the previous example to obtain a trifluorocomposition by replacing the PS-445 second component with an equivalentweight percentage of PLY-7801, marketed by McGhan Nusil Corp. ofCarpinteria, Calif., and replacing the VM&P Naphtha with an equivalentweight percentage of 1,1,1-trichloroethane.

EXAMPLE 16

We modified Coating LS of the previous example to obtain a dual-curecomposition by replacing the PS-445 second component with an equivalentweight percentage of PS-344.5, and replacing the PS-120 cross-linkingagent with an equivalent weight percentage of PS-128; all componentswere supplied by Huls. PS-344.5 is a silanol-terminatedpolydimethylsiloxane compound, and PS-128 is amethyldimethoxy-terminated polymethylhydrosiloxane compound. Moistureactivates the methoxy-silanol condensation reaction, which benefits fromthe acidic by-products of the chloroplatinic acid catalyst (PC-072) thataccelerates the addition-cure reaction between the vinyl groups of thegum component and the hydrosiloxane groups of the cross-linkingmolecules.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed. For example, for production-scale coatingpreparation, it may be advantageous to add the second component directlyto the gum/pigment dispersion to produce an easily handled paste;alternatively, it might prove desirable to add the solvent directly tothe gum/pigment dispersion, or to add both the solvent and secondcomponent simultaneously.

What is claimed is:
 1. A recording medium capable of being imaged usingablative discharge, comprising a substrate having applied to at leastone surface thereof a composition comprising a cured polymeric siliconematrix consisting essentially of the reaction product of:a. a firstpolysiloxane component having a molecular weight in excess of 300,000and comprising substantially linear chains of substituted siloxaneunits, some of which units contain, as substituents, at least onereactive functional group, each functional group being randomlyinterspersed in each first-component chain; b. a second polysiloxanecomponent having a molecular weight in excess of 5,000 but no greaterthan 70,000, and comprising substantially linear chains of substitutedsiloxane units, each of which is capped by terminal siloxane units thateach contain, as substituents, at least one reactive functional group;and c. a cross-linking component having a molecular weight no greaterthan 5,000 and containing functional groups reactive reactive with atleast some of the functional groups of the first component or of thesecond component or of both components; wherein d. the relative weightproportion of the first component to the second component is at least10% and no more than 90%.
 2. The recording medium of claim 1 whereinnone of the functional groups of the first component are reactive withany of the functional groups of the second component.
 3. The recordingmedium of claim 2 wherein the functional groups of the first and secondcomponents are reactive with the functional groups of the cross-linkingcomponent.
 4. The recording medium of claim 3 wherein the functionalgroups of the first and second components react with the functionalgroups of the cross-linking component via addition cure.
 5. Therecording medium of claim 3 wherein the functional groups of the firstand second components are moieties and the functional groups of thecross-linking component are silicon-hydride species.
 6. The recordingmedium of claim 3 wherein the functional groups of the first and secondcomponents are silicon-hydride moieties and the functional groups of thecross-linking component are α-olefinic moieties.
 7. The recording mediumof claim 2 wherein functional groups of only one of the first and secondcomponents are reactive with the functional groups of the cross-linkingcomponent.
 8. The recording medium of claim 7 wherein the functionalgroups of one component react with the functional groups of thecross-linking component via addition cure.
 9. The recording medium ofclaim 7 wherein the functional groups of the first component areselected from the group consisting of acrylate, methacrylate and epoxyspecies, the functional groups of the second component are α-olefinicspecies, and the functional groups of the cross-linking component aresilicon-hydride species.
 10. The recording medium of claim 7 wherein thefunctional groups of the first component are selected from the groupconsisting of acrylate, methacrylate and epoxy species, the functionalgroups of the second component are silicon-hydride species, and thefunctional groups of the cross-linking component are α-olefinic species.11. The recording medium of claim 7 wherein the functional groups of thefirst component are α-olefinic species, the functional groups of thesecond component are selected from the group consisting of acrylate,methacrylate and epoxy species, and the functional groups of thecross-linking component are silicon-hydride species.
 12. The recordingmedium of claim 7 wherein the functional groups of the first componentare silicon-hydride species, the functional groups of the secondcomponent are selected from the group consisting of acrylate,methacrylate and epoxy species, and the functional groups of thecross-linking component are α-olefinic species.
 13. The recording mediumof claim 7 wherein the functional groups unreactive with the functionalgroups of the cross-linking component are reactive with one another. 14.The recording medium of claim 13 wherein the functional groupsunreactive with the functional groups of the cross-linking componentreact with one another via exposure to electron-beam or ultravioletradiation.
 15. The recording medium of claim 13 wherein the functionalgroups unreactive with the functional groups of the cross-linkingcomponent are selected from the group consisting of acrylate,methacrylate and epoxy species.
 16. The recording medium of claim 7wherein the functional groups of one component react with the functionalgroups of the cross-linking component via condensation cure.
 17. Therecording medium of claim 7 wherein the functional groups of onecomponent react with the functional groups of the cross-linkingcomponent via moisture cure.
 18. The recording medium of claim 1 whereinthe substrate comprises a strong oleophilic material and a metal layer.19. The recording medium of claim 1 wherein the substrate comprises astrong oleophilic material and a radiation-absorptive layer.
 20. Therecording medium of claim 1 wherein the silicone composition furthercomprises at least one pigment dispersed therein.
 21. The recordingmedium of claim 18 wherein the substrate, metal layer and siliconecomposition cooperate to form a planographic printing plate.
 22. Therecording medium of claim 18 wherein the substrate, metal layer andsilicone composition cooperate to form a planographic printing plate.23. The recording medium of claim 1 wherein the mole ratio of siloxaneunits containing a functional group to those which do not ranges fromapproximately 0.1 to 1.0 percent.
 24. The recording medium of claim 1wherein the siloxane units include dimethylsiloxane units.
 25. Therecording medium of claim 1 wherein at least 70% of the substitutedsiloxane units are dimethylsiloxane units.
 26. The recording medium ofclaim 1 wherein the first molecular component has a molecular weight ofat least 500,000.
 27. The recording medium of claim 1 wherein thesiloxane units also include higher alkylmethylsiloxane units, the alkylgroup having up to 18 carbons.
 28. The recording medium of claim 27wherein the siloxane units also include phenylmethylsiloxane units. 29.The recording medium of claim 1 wherein the siloxane units also includediphenylsiloxane units.
 30. The recording medium of claim 1 wherein thesiloxane units also include 3,3,3-trifluoropropylmethylsiloxane units.31. The recording medium of claim 30 wherein up to 80% of thesubstituted siloxane units are 3,3,3-trifluoropropylmethylsiloxaneunits.
 32. The recording medium of claim 28 wherein the proportion ofphenylmethylsiloxane units to dimethylsiloxane units does not exceed20%.
 33. The recording medium of claim 29 wherein the proportion ofdiphenylsiloxane units to dimethylsiloxane units does not exceed 20%.34. The recording medium of claim 19 wherein the radiation-absorptivelayer contains unreacted acrylate or methacrylate functional groups, andone of the two base-polymer components also contains acrylate ormethacrylate functional groups.
 35. The recording medium of claim 19wherein the radiation-absorptive layer contains unreacted α-olefinicgroups, and the cross-linking component contains silicon-hydridefunctional groups.